1. Field of the Invention
The present invention generally relates to an optical disk as one kind of optical recording media on which signals, such as image information, audio information etc. are recorded, to a method of recording the optical disk and to an optical pickup apparatus for reproducing signals recorded on the optical disks. More particularly, the present invention is concerned with a recording format of signal pits formed on an optical disk. Furthermore, the present invention relates to an arrangement of a light path and a light detection means in an optical pickup apparatus for reproducing signals recorded on such an optical disk.
2. Description of the Related Art
An optical disk is a recording medium for recording a large amount of digital and analog information, such as image and audio information. A reflection type reproducing system can be used for reproducing the signals recorded on the optical disk. In the reflection type reproducing system, laser beams are projected onto the optical disk, and recorded information is reproduced from light beams reflected by a signal recording plane of the optical disk. The reflection type reproducing system can be configured by a simple structure and made compact.
The reflection type optical disks are categorized into a ROM optical disk, a write once optical disk or a recordable optical disk. In any of these types, information is recorded in the form of pits formed on the recording plane of the optical disk. Pits are spirally arranged in a line from an inner circumference of the disk to an outer circumference thereof. One signal track is defined as one turn of pits. A process for positioning the light beams on the optical disk is carried out for each signal track. It will be noted that the term "pit" originally means a hole or recess, but here includes a case where the recording of information is performed without a change in the shape of the recording plane. In magneto-optical recording or phase change recording, information is recorded without change in the shape of the recording plane.
This type of optical pickup apparatus is provided with, for example, a lateral mode or a single transverse mode semiconductor laser and a photodetector. The semiconductor laser forms a point light source, which has a light emitting point having a diameter of approximately 0.1 .mu.m. The half mirror may be disposed to separate a projection light emitted from the semiconductor laser and directed to an optical disk, from a reflected light therefrom. The photodetector detects the reflected light. The laser beams emitted from the semiconductor laser are reflected by the half mirror, and form an image on the recording plane of the optical disk by means of the objective lens. The wavelength .lambda. of the semiconductor laser and the numerical aperture NA are selected so that .lambda./NA is greater than 0.1 .mu.m. Hence, the size of the spot on the optical disk is limited to .lambda./NA due to a diffraction limit.
As has been described previously, information is recorded on the optical disk in the form of pits, which cause optical change. The explanation will be made hereinbelow as for the case that the pits formed on the recording plane cause a change in the refractive index.
In this case, the recording of information is performed by sensitizing pigment contained in a recording layer or changing the state of the recording layer to a crystal state or an amorphous state, so that areas having different refractive index values are formed on the recording plane. The reflected light from the recording plane, which has different intensity levels due to the differences in the refractive index, is converged by the objective lens. Some of the reflected light passes through the half mirror. At this time, the half mirror causes the reflected light to have an astigmatism characteristic. Accordingly, the reflected light from the half mirror has different converting positions in the longitudinal and lateral directions perpendicular to the light axis. The photodetector is located between these different converging positions in order to enable focusing operation.
As shown in FIG. 1A to 1C, this type of photodetector 101 consists of four equally divided photodiodes. FIG. 1A shows a case where the focusing position is far from the recording plane, and FIG. 1C shows a case where the focusing position is close to the recording plane. FIG. 1B shows a case where the focusing position is on the recording plane. The position of the photodetector 101 is adjusted so that the light spot shown in FIG. 1B is obtained. A focusing error can be detected by calculating the difference between diagonal components of the light spot formed on the photodiodes. The focusing control is performed based on the focusing error thus calculated. It will be noted that the light spot identical to that formed on the photodiodes is not formed on the optical disk. Namely, the light spot formed on the photodiode is a so-called far field pattern, which is shifted by a half of the difference between the converging positions. Japanese Patent Publication No. 52-50131 discloses a structure in which a signal for use in focusing is detected at the focusing position. With this structure, it becomes possible to suppress the movement of the light spot on the photodetector.
In the above described type of optical pickup apparatus, if the distance between the adjacent signal tracks is made reduced in order to increase the signal recording density, the light spot formed on the signal track becomes to be significantly affected by a change in the reflectance of adjacent signal tracks. Hence, leakage of light from the adjacent signal tracks takes place, and the S/N ratio of the reproduced signal is degraded.
Japanese Patent Application Laid-Open Publication No. 57-58248 discloses a high-density reading method, in which a plurality of light sources are used. Three adjacent signal tracks are projected by these light sources, and far-field patterns respectively formed by the light sources are detected by photodetectors. The output signals of the photodetectors corresponding to the respective far-field patterns are mutually subtracted from each other in accordance with a leak rate measured beforehand. Using the results of the above calculation, the quantity of leakage is to be reduced. However, the above method needs a light spot having a size corresponding to the diffraction limit. If the interval between the adjacent signal tracks is made smaller than the limited size of the light spot, the influence of the outer signal track will take place. As a result, the above method does not bring about the great improvements.
As described above, it is very difficult to improve the information recording density on the optical disk by narrowing the distance between the adjacent signal tracks.
On the other hand, it is required that information be more rapidly read as an increased amount of information is recorded on the optical disk. Particularly, it is necessary to use a broadband recording and reproducing system capable of recording and reproducing high-frequency signals in order to record and reproduce information used in a high definition television system, for example. In order to realize such a broadband recording and reproducing system, it has been attempted to more finely record information on the optical disk, and read an increased number of pits per unit time, or per unit length.
However, the fine recording of information needs a smaller minimum pit length on the optical disk. In order to reproduce the original images at such a required resolution level, it becomes necessary to broad a spatial frequency band of the optical pickup apparatus used for reproduction. For this requirement, it is necessary to shorten the wavelength of the light source or increase the numerical aperture of the objective lens. The currently available semiconductor laser devices cannot emit light having such a required wavelength. Thus, it is necessary to use a gas laser or a laser using a non-linear optical element. An increased numerical aperture needs a flatter optical disk or an optical disk having a uniform disk surface. However, It is very difficult to produce such optical disks. As a result, it is very difficult to broad the spatial frequency band for reproduction.
It may be possible to rotate the optical disk at a higher speed in order to reproduce signals in a high-frequency band. However, in this case it is necessary to use a more powerful motor for rotating the optical disk. In addition, it is difficult to control the focusing position following an increased revolution of the motor.